The announcement comes at a time of crumbling ties between the North and South, with Pyongyang blaming Seoul for the outbreak of Covid-19 in its territory and conducting a record number of weapons tests this year.

The law will allow North Korea to carry out a preventive nuclear strike "automatically" and "immediately to destroy hostile forces," when a foreign country poses an imminent threat to Pyongyang, the official Korean Central News Agency (KCNA) said.

With the newly enacted law, "the status of our country as a nuclear weapons state has become irreversible," leader Kim Jong Un said, according to KCNA.

A blitz of North Korean weapons tests since January included firing an intercontinental ballistic missile at full range for the first time since 2017.

Washington and South Korean officials have repeatedly warned that the North is preparing to carry out what would be its seventh nuclear test.